Guided wave transmission



April 16, 1940; A. P. KING GUIDED WAVE TRANSIISSION Filed June 18. 1'93?2 Sheets-Sheet 1 M .lw u H I m W N E m I H B w 0 0 0 0 w w m M m m w 6 w2 2 EUQQ Rum Emfiumai t $2M: x

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INVENTOR AP; KING I00 INTERNAL DIAMETER IN CM.

ATTORNV Patented Apr. 16, 1940 GUIDED WAVE TRANSMISSION Archie P. King,Red Bank, N. 1., assignor to Bell Telephone Laboratories,

Incorporated,

New York, N. Y., a corporation of New York Application June 18, 1937,Serial No. 148,879 12 Claims. (01. 178-44) This invention relates toelectromagnetic wave attenuators and more particularly to attenuatorsfor use in a system for dielectrically guided waves.

Dielectric guide systems of various kinds have been described in somedetail heretofore in such applications for Letters Patent as those of G.C. Southworth which issued on September 13, 1938 as U. S. Patents No.2,129,711 and No. 2,129,712

10 and that of S. A. Schelkunofl which issued on February 21, 1939, asU. S. Patent No. 2,147,717, and in the papers by J. R. Carson et al. andSchelkunoif appearing in the April 1936 issue of the Bell SystemTechnical Journal. The dielectric guide itself has taken a wide varietyof forms, but typical of guides disclosed heretofore is one consistingof a rod of dielectric material and another consisting essentially of ametallic pipe containing a solid or gaseous dielectric medium.

A form of dielectric guide that lends itself well to the purposes inhand is one consisting of a metallic pipe, evacuated or filled with air,and it is in terms of such a guide that my invention will be described.It is to be understood, however, that this is for illustrative purposesonly and that the invention is not to be limited to this specific formof guide.

Dielectrically guided wave transmission as disclosed in the applicationsand'publication cited above, is unique in several respects. In the firstplace it is evident that the provision of separate conducting paths forthe go-and-return fiow of conduction current is not an essentialcharacteristic whereas in conventional guided wave systems knownheretofore it is. Secondly, in each instance it has been observed thatthe guide presents the attenuation characteristic of a highpass filter,that is, there is a certain critical or cut-oil frequency separating thepropagation range from a lower frequency range of zero or highlyattenuated transmission. Moreover, it has been found that the criticalfrequency and the phase velocity of dielectrically guided-waves are bothfunctions of the transverse dimensions of the guide.

Dielectrically guided waves are capable of transmission in anindefinitely large number of forms or types, each type beingdistinguished by the characteristic spacial distribution andinterrelation of the component electric and magnetic fields comprisingthe waves.

Although as noted, there are an indefinite number of types ofdielectrically guided waves, 55 it has been found that they fall intoeither of two broad classes.

In the one class, assuming now for the sake of simplicity that the guideis in the form of a metallic pipe, the electric component of the wave istransverse to the pipe and at no point does it have a longitudinalcomponent excepting as the pipe is not quite a perfect conductor. Themagnetic component, on the other hand, has both transverse andlongitudinal components. This class will be designated as transverseelectric waves or TE waves. In the other class, the magnetic componentis transverse to the pipe and at no point does it have a longitudinalcomponent, but the electric component has in general both transverse andlongitudinal components. This class will be designated as transversemagnetic waves or TM waves.

The various possible types of v dielectrically guided waves in each ofthese two classes may be identified and distinguished from each other bytheir order and by their mode of propagation. The order of the wave isdetermined by the manner in which the field intensity variescircumferentially around the axis of the guide, whereas the mode isdetermined by the manner of its variation with distance from the axis ofthe guide. Reference is made here to the Schelkunoff application, supra,for a more complete discussion of this matter of mode and order. Theusual convention is herein adopted of designating a TE wave by Hnm,where n represents the order and m the mode. Similarly a TM wave of thenth order and mth mode will be represented by Enm.

- The purpose of this invention is that of providing suitableattenuating devices for wave guide systems carrying one or another ofthe different types of dielectrically guided waves described above. Morespecifically the purpose of the invention is to provide a seriesattenuating device as distinguished from a shunt attenuating device forsuch systems. Still a further purpose is to provide attenuators whichmay be continuously adjustable to yield any desired degree ofattenuation. Other purposes will appear presently.

The invention will be better understood by reference to the followingspecification and the accompanying drawings in which:

Figs. 1 and 2 arecurves showing certain characteristics of wave guides;and

Figs. 3 to 12 show various forms of attenuators for carrying out myinvention.

Referring more specifically to Fig. 1, there are shown the attenuationcurves of the four principal types of dielectrically guided waves as afunction of the ratio of the operating free space wave-length A to theinternal diameter d of the guiding structure. The values are calculatedfor a cylindrical copper guide with air dielectric, and the criticalratios corresponding to thecut-ofl frequencies are indicated by thearrows. In Fig. 2 the curves of Fig. 1 are redrawn to show the relationbetween attenuation and diameter at a constant wave-length 7\.=l5centimeters, the curves for convenience being plotted on a logarithmicscale.

Fig. 2 indicates that the attenuation per centimeter length of guide isextremely low except near cut-oil. The transmission loss can beincreased, however, by increasing the wall resistivity of the wave guideand an important part of my invention resides in the use of this fact.For a wave guide as ordinarily used, with a metallic wall and a diameternot too near the cut-oil, attenuations of the order of 10- and 10-decibels per centimeter prevail. Since the rate of attenuation is thusvery low, it would require a section of guide of inconveniently greatlength to produce an appreciable loss. To obtain transmission losses offrom 0.1 to 10 decibels per centimeter and thus to realize substantialtransmission losses in an attenuator of moderate length, is an object ofthis invention.

Fig. 3 shows a form of attenuator which is adapted for a large varietyof types of dielectrically guided waves such, for specific example, asan H11 wave. Mounted between two highly conducting guide sections, W1and W3, is the high resistance section W2. The length I of this cylinderexposed to the passing wave is adjustable by sliding W3 longitudinally,thus varying the amount of attenuation. The resistance section mayconsist of a large variety of resistance elements such, for example, asa piece of blotting paper impregnated with colloidal graphite. With sucha system a wave propagated through the guide will sufl'er loss of powerat a fairly high rate while passing through the section W2.

In such a section of attenuator there may be a tendency for the waveproper to leak out through the high resistance wall. This leakage may beconsidered as part of the attenuation. On the other hand, any suchleakage is frequently undesirable because of the external effectsproduced. It may be very materially reduced by applying an externalsheath S of copper foil although this will have a tendency to reduce theattenuation proper, because of the preference of the wave to travelalong the low resistance sheath.

However, by increasing the thickness of the resistance element, as byusing several layers of impregnated blotting paper, the leakage factorcan be largely reduced without excessive reduction of the attenuatingfactor. As a general proposition it may be stated that high conductivityattenuator walls will require thin-walled sheaths and low conductivityattenuator walls will require thick-walled sheaths to reduce theexternal field to a desired low value.

The attenuations referred to above can, I find, be increased by areduction in the diameter of the power absorbing section of the line.Thus, in a particular instance, for a wave-length of fifteen centimetersa decrease in the diameter from 12.6 centimeters to 10 centimetersincreased the attenuation per unit length by a factor of 2.5 whereas adecrease to nine centimeters diameter increased the attentuation by afactor of 6. This may be readily understood by reference to theattenuation curves of Fig. 1 and the principle involved is oneapplicable to my invention which will be disclosed in humor detail inthis specificat on.

It is to be noted that the presence of a high resistivity guide sectionmay set up a substantial reflected wave. This is in part due to thechange in characteristic impedance of that section of the linerepresented by the attenuator, for such characteristic impedance isdependent to some extent on the resistance-of the guide section. Forthis reason it will frequently be desirable to make the resistivity ofthe guide of a value which will not represent too large a departure inimpedance characteristic from that of the main portion of the guide andthen to use a longer length of attenuator section to obtain the desiredattenuation. 0r again the large change in characteristic impedance maybe avoided by a change in ghe diameter of the attenuator portion of the8H! e.

In this method of wave propagation it is convenient to speak of' thecritical diameter'of a guide, where the critical diameter is defined asthe smallest diameter which will permit the propagation of a wave of agiven type and length in a perfectly conducting guide. Such criticaldiameters may curves of Fig. l for those types of are there represented.

Where the material comprising a pipe guide is not a substantiallyperfect conductor, transmission does not entirely cease when the wavefrequency or the pipe diameter is reduced below the critical frequencyor critical diameter, respectively, as hereinbefore defined, so that asmaller guide than indicated by theoretical cutoff may transmit a wave.The eflect, I find, becomes more pronounced as the resistivity of thehollow conductor wall increases and the corresponding attenuation curvenear the cut-off becomes appreciably less sharp. I make use of thisfeature in the manner shown in Fig. 4, in which a thin piece of metalsuch as phosphor-bronze may be rolled into a tube l5 and held centrallyin the main tube by a metallic adjustable iris I6. As the diameter ofthe tube I5 is decreased by means of the iris l6, one approaches moreclosely to the cut-01f value for this tube, the attenuation rising quiterapidly but not so rapidly as indicated by the curves of Fig. 1. Over areasonably wide range the change in attenuation is substantial. Thus Ihave found it feasible to obtain in this manner attenuations as high as5 decibels per centimeter.

Further control of the attenuation may be obtained by suitable choi e ofmaterial so far as its resistivity or other physical properties areconcerned. Thus if the pipe 1570f Fig. 4 is to be of fixed diameter, asshown in Fig. 5, then this pipe may be chosen of one material oranother. I find, for example, that iron, because of its permeability,exhibits a higher attenuation than tubes of non-permeable materials ofcomparable resistivity.

As a further illustration of the modes of application of my invention itis to be noted that a desired value of attenuation may be obtained froma plurality of units of fixed attenuating characteristics. Thus in Fig.6 the attenuator consists of three sections, A1, A2 and A3. From anassortment of such elements and by combining them in the mannerindicated, a large number of values may be obtained. In the event ofwaves which impedance discontinuity due to such element one may providean adjustable side chamber or an be readily obtained from the a aromasequivalent iris, these serving as reactive elements to cancel anyimpedance discontinuity or' to vary the amount of attenuation. Theresistivity, thickness and diameter of the elements may be constant asshown in Fig. 6 or they may vary as shown in Fig. 7, to maintaincharacteristic impedarice. In such cases also the resistivity may varyradially or longitudinally, or the thickness, length, diameter andresistivity may vary in any desired manner. Furthermore, the diameter ofthe attenuating section may be either above or below the theoreticalcut-oi! value. Using a reduced diameter section of guide to produceincreased loss, as in Fig.-'l, one may make a con-.

nection either with the tapered joint as at C or an abrupt joint as atG. The kind of joint employed will be largely dependent upon theoperating conditions. Obviously also external sheaths 8 shown in Fig. 6and Fig. 7 may be used.

An attenuator which makes use of the adjust ment of length to introducethe desired loss is shown in Fig. 8. Here the attenuating section We ismounted between two guides fixed in position and separated by the gap L.The attenuator comprises two sections of guide one portion A1 being ofhigh loss and the other portion A: being highly conductive. The amountof attenuation is adjusted by sliding W2 longitudinally over theconnecting guides thus varying the length l of the high loss sectionexposed to the waves.

Here. again the resistivity of A1 may be constant or may vary radiallyor longitudinally, or

may be varied with thickness and the thickness,

of the wall may vary longitudinally. An external sheath S may be used ifdesired.

Fig. 9 illustrates a means of attenuation adjustment with a guidesection W1 operated at a frequency below the cut-off frequency. Slidingin the section W2 and of approximately the same length is an adjustabledielectric rod or plug E. The dielectric constant of the rod issufllciently high so that the portion of W: containing the dielectricwill be above cut-off and will thus propagate the' wave with relativelylow attenuation. For the unoccupied portion, however, the attenuation ishigh. If W: is of constant diameter or cross-section the attenuationinserted in the circuit is proportional to l, which is that section ofW: with air dielectric. One or both ends of the dielectric may bespecially shaped as indicated by P to reduce distortion or impedancediscontinuity- This adjustable featuremay obviously be used withadditional sections of W: or in conjunction with the attenuator of Fig.'7 to provide an extended range of attenuation.

In Fig. 10 there is shown an attenuator making use of a guide length ofcontinuously variable cross-sectional area. This type, which has beenpreviously described in a somewhat simplified form in Fig. 4, consistsof a variable diameter tube W: and the adjustable irises I1 and 1:. The

tube W: is a thin flexible metallic sheath rolled into a cylindricaltube whose spring tension tends to unwind or extend the diameter of thetube.

. This tension makes the tube conform to and cross-section whose areacould readily be changed by varying the spacing between opposite sides.Fig. 11 indicates a method of varying the power transmission in anattenuator by means of a guide material whose resistivity may be alteredby passage of electric current or whose permeabilin resistivity arethyrite, boron, silver sulphide,

copper oxide, etc. It W: is the resistive section the current flow maybe longitudinal or circumferential, or both.

Fig. 12 shows a method for varying the permeability of a magneticmaterial used as an attenuator section. This may be accomplished bymeans of a solenoid wound over the permeable part and coaxial with theguide. The permeability of We can be varied by changing the current flowthrough the coil. The efliciency oi the magnetic circuit may, of course,be increased by using a closed magnetic circuit as shown.

It is to be understood that any suitable reacting elements such as aside-chamber or iris may be used to compensate the reactance orimpedance discontinuities in any of these attenuators.

While the invention has been described thus far in terms of one or moreseries absorbing elements, it is to be understood that one may combinethem in various ways with shunt absorbing elements, such as thosedescribed in the copending application of A. E. Bowen, Serial No.148,839, filed of even date herewith.

What is claimed is: 1. In an electrical transmission system, a shieldedtransmission structure for propagating ultra-high frequency waves withlow attenuation, an attenuator comprising a short length of conductivepipe electrically interposed in tandem relation in said transmissionstructure for the transmission of said waves in the form ofdielectrically guided waves, the transverse dimensions of said pipebeing so related to the frequency of said waves that said frequency isat least approximately the cut-01f frequency of said" pipe whereby theenergypf said waves is in part dissipated in the form of heat in saidattenuator and the rate of attenuation in said pipe is large :omparedwith that in said transmission strucure.

2., In a dielectric wave guide system, an attenuator for dielectricallyguided waves comprising a section of guide of adjustable loss value,said section comprising a sleeve adapted to slide over the ends of twoadjacent guide sections, the portion of the sleeve which is exposed tothe passing wave being capable of adjustment, the inner face of saidsleeve comprising resistance material so distributed that a variableamount thereof can be exposed by effecting said adjustmen 3. In adielectric wave guide system, an attenuator for dielectrically guidedwaves comprising a section of'guide of adjustable loss value,

said section comprisinga sleeve to slide over two adjacent guidesections, the sleeve having one portion of high loss value and the otherof low loss value.

. 4. In a dielectric wave guide system, an attenuator for dielectricallyded waves, an attenuator comprising a section of guide of adjustableloss value, said section comprising a sleeve to slide over two adjacentguide sections, the sleeve having one portion of high loss value and theother of low loss value and being adapted to slide longitudinally toexpose a greater or smaller portion of the high loss section to thepassing wave.

5. In combination, a hollow metallic pipe constituting a guide for thetransmission of 'dielectrically guided waves, an attenuator interposedin said guidecomprising a pipe section having a high rate ofattenuation, said pipe section being composed of a resistive materialfor dissipating the energy of said waves passing through it, and meansfor adjustably controlling the resistivity of said material whereby theattenuation introduced by said pipe section can be regulated.

6. In a dielectric wave guide system, an attenuator for dielectricallyguided waves comprising a section of high loss guide, said sectioncomprising a material the resisticity of which is a function oftemperature, and means for adjustably controlling the temperature ofsaid material whereby the loss introduced by said attenuator can beregulated.

7. A wave guide comprising a metallic pipe, means for transmittingdielectrically guided waves therethrough, and an attenuator for saidwaves comprising a short section of said pipe a transverse dimension ofwhich is so related to the frequency of said waves that said dimensionlies between the value for cut-ofi and the value at which transmissionceases, whereby said waves are transmitted through said attenuator butwith reduced amplitude.

8. A combination in accordance with claim 7 comprising a body ofdielectric material having a dielectric constant greater than unity andadapted to be advanced into said section of pipe.

9. A system comprising a metallic pipe for the transmission ofdielectrically guided waves and a device interposed in said pipecomprising a pipelike section having a high rate of attenuation for saidwaves, the total amount of attenuation introduced by said device beingdependent on- (a) the length of said section, (b) the resistivity of thematerial comprising it, (c) its transverse dimensions and (d) thedielectric coeflicient of the dielectric medium contained within it, andmeans for changing at least one of the parameters (a), (b), (c), (d),whereby the total attenuation introduced by said deviceis changed.

10. In a system for the transmission of dielectrically guided waves, awaveguide comprising a metallic pipe, and a variableattenuatorcomprising a section of: said pipe at least a portion a of the wall ofwhich comprises material of high resistivity and means for adjusting theamount of said material that is exposed to waves transmit-' ted throughsaid pipe, whereby said waves proceed beyond said attenuator withreducedamplitude. g

-11. In combination with a metallic pipe for the transmission ofdielectrically guided waves, a

localized device comprising material of high resistivity disposed in thepath of said waves for attenuating them and means for adjustablycontrolling the resistivity of said material whereby to adjust theattenuation suflered by said waves. 12. In a wave guide comprising ametallic pipe containing a dielectric medium for the transmission ofdielectrically guided waves, avariable' attenuator comprising a sectionof said pipe including means for adjustably fixing the transmissioncut-ofi frequency of at least a portion thereof, the frequency of saidwaves being approximately said cut-oif frequency whereby a change in thelattereflects a disproportionately large change in the amplitude of thewaves transmitted through said attenuator.

'ancrmz: P. KING.

